I'm a little weary of using acid cleaners on gelcoat unless the gelcoat has rust stains. Gelcoat has free silica gel particles and acid will etch and erode these particles leaving the surface excessively porous and succeptible to future staining. Oxalic acid is the best acid for removing rust stains, so if you have rust stains, use a product which contains it. Otherwise I'd stick with a rubbing compound. My work experience is in professional refinishing, including on boats and yachts

Jimbo